From an interview with Fr. Thomas Rosica:
Q: When did we first start speaking of angels in the Catholic Christian tradition?
A: Angels are found among the four Western "religions of the book": Zoroastrianism, Judaism, Christianity and Islam.
While they were an element of Christian piety from the beginning ... the study of angels, angelology, took off in the early sixth century, when the theologian Dionysius the Areopagite first classified them. His celestial hierarchy would later be elaborated by St. Thomas Aquinas, the Italian Dominican scholar of the 13th century who was nicknamed the "dumb ox" by his classmates, but the "angelic doctor" by the Church, not because of his holy life, but rather because of his profound teaching about angels...
... They are spiritual realities that reveal themselves to women and men in moments of vision, and that also break in on human consciousness in dreams, flooding it with awareness and wonder and fear.
... The notion of angels is [to be] mysteriously in the presence of God on our behalf, and simultaneously with us on our behalf. What a tender gesture of our loving Creator! Guardian angels provide free, womb-to-tomb guidance — lighting and guarding, ruling and guiding ... all the days of our lives.
Angels move our imaginations with good thoughts and impulses, and impel us toward goodness ... through secret impulse, intuition, without the benefit of [our] actually seeing or hearing them. They pray with us and for us, and in transporting our prayers to God, they may alter them ever so slightly to make them more perfect. They protect us in times of danger, in the physical as well as in the spiritual life, because not all is sweetness and light here below.
The appearance of angels in the Old and New Testaments, as well as in our lives, is consistent with a minor actor in a major play: They have one line to deliver, or a task to perform; they do it, and exit promptly. It is God who always takes the credit for their interventions and successes...
... If the angels teach us anything, they show us what it means to put on the mind of Christ. What a great privilege is theirs, to stand constantly in God's presence, to feast their eyes on Jesus, to know his face and even more, his mind. They look upon the world, and on each of us, with the mind of Christ.
To truly love someone is not only to adore their face and their external reality, but to enter their mind and heart. To have the mind of Christ is not a boast but a prayer, and the prayer is that we, more and more, learn to think his thoughts and to see the world around us through his eyes. We have not only the spirit, the love and the strength of Christ. We also have been given his mind.
... Second, the angels teach us about simplicity, about delighting in God's presence. Responding to the question of who was the greatest, Jesus called a child, whom he put among them, and said, "Truly, I tell you, unless you change and become like children, you will never enter the kingdom of heaven. Whoever becomes humble like this child is the greatest in the kingdom of heaven. Whoever welcomes one such child in my name welcomes me. Take care that you do not despise one of these little ones; for, I tell you, in heaven their angels continually see the face of my Father in heaven."
St. Augustine captured this well when he said, "It was pride that changed angels into devils; it is humility that makes men as angels." Children and angels know how to delight and how to rejoice! In the midst of our busy lives, I fear that we have lost the art of delighting and rejoicing. How often do we focus on our disappointments, rather than our delights!
Third, the angels invite us to become angels and messengers for one another. For what is ultimately their role—to be messengers, bearers of words of consolation, hope, peace, joy, protection ... to remind others of the beauty and consolation of God's presence ... to invite us ever more deeply into the mystery of God ... to mirror God and God's glory to others ... to gently lead others to God.
Thomas